https://news.ycombinator.com/item?id=10957020
I think, usable day-to-day, I'd say you're down to Haiku, MorphOS, Genode, MINIX 3, and/or A2 Bluebottle. Haiku is a BeOS clone. MorphOS is one of last Amiga's that looks pretty awesome. Genode OS is a security-oriented, microkernel architecture that's using UNIX for bootstrapping but doesn't inherently need it. MINIX 3 similarly bootstrapping on NetBSD but adds microkernels, user-mode drivers, and self-healing functions. A2 Bluebottle is most featured version of Oberon OS in safe, GC'd language. Runs fast.
The usability of these and third party software available vary considerably. One recommendation I have across the board is to back up your data with a boot disc onto external media. Do that often. Reason being, any project with few developers + few users + bare metal is going to have issues to resolve that long-term projects will have already knocked out.
BTW, prior to SqueakNOS we implemented this: http://swain.webframe.org/squeak/floppy/ (using Linux and modifying Squeak to work with SVGALib instead of X) in just 900mb inspired in QNX Demo Disk: http://toastytech.com/guis/qnxdemo.html
It's neat that it was the thing that inspired one of your Squeak projects. Is SqueakNOS usable day-to-day in any console desktop or server appliance context? Key stuff reliable yet?